The invention relates to an automatic silk-screen printing system having two oppositely working scraping blades which are each fixed to a blade support for depositing a silk-screen printing product in accordance with a given configuration on a support to be coated via a screen to be positioned on said support, said screen being perforated in accordance with said configuration.
The invention is applicable to the silk-screen printing technique in general, and more particularly to the technique which consists of depositing a soldering paste on contact areas of a printed circuit board or a ceramic board for subsequent soldering of the components in accordance with a melting process which is akin to braze-welding, a technique particularly adapted to surface-mounted components. In this particular preferred application, the soldering paste which constitutes the silk-screen printing product is constituted, for example by pellets of a Pb-Sn alloy having a diameter of several microns and agglomerated by means of a binder giving this paste thixotropic qualities. The screen is made of a thin leaf of stainless steel of 180 .mu.m or 150 .mu.m, provided with cavities arranged above the contact areas on the printed circuit board or the ceramic board to be coated.
To automate the process, two blades working in opposition are used. Each blade of flexible steel is fixed to a support and the two supports of the blade are firmly tightened by means of a clamp so that the blades enclose a mutual angle of the order of 90 degrees and an angle of approximately 45 degrees with respect to the screen. The clamp itself forms part of a carriage with respect to which it can move approximately 10 degrees to the one or the other side of a median position in which the blades substantially constitute the same angle with respect to the screen. Moreover, a vertical translation movement of the clamp with respect to the carriage allows the one or the other blade to engage the screen with a certain nominal predetermined pressure, after which the horizontal translation movement with which the silk-screen printing operation is carried out is effected on a support to be coated, such as a printed circuit board. At the end of the process, the clamp is raised and an unused soldering paste is left on the screen by the blade which was in action and subsequently the clamp performs a tilting movement so as to raise again the first blade which was in action and to lower it to a level which is smaller than that of the first blade which was raised and inactive during the preceding passage of the carriage. The clamp is subsequently lowered until the second blade engages the screen with said predetermined pressure, while the translation movement of the carriage restarts in a direction reverse to the previous direction and the solder paste left by the first blade is taken up upon the passage by the second blade, and a new support (printed circuit board) is coated. When the horizontal course of the carriage ends, the same operations of raising, tilting and lowering start again symmetrically, while the two blades are interchanged and the cycle recommences. During these operations some of the silk-screen printing product (which will hereinafter be referred to more generally as cream so as to designate either an ink or a paste) is used up by the product to be coated and, at each passage of the carriage, a part laterally brims over on the screen at the two extremities of the blade. Consequently, extra cream must be periodically added on the screen between the two blades after a certain number of blade passages.
Such a system has the drawback that the cream which has brimmed over at the extremities of the blades forms a bead which is not always taken up by the blades, which bead thickens and progressively dries up. Under these conditions it is impossible to control the quantities of the product (cream) which are effectively used and the lost quantities of the product. The cream which is involuntarily evacuated on the sides loses its qualities of viscosity due to evaporation of the solvents and the binders which it contains; after a delay which is short enough to correspond to approximately twenty passages of the blades, this cream is definitively unusable. In the prior art, this displacement of the cream towards the working zone requires a frequent manual intervention to rearrange the product which has brimmed over, which is not very satisfactory for an automatic system.
Another drawback of the prior art mentioned above is that the use of a system of automatically supplying the silk-screen printing cream is risky because the cream which has become unusable is not easily taken into account because it is difficult to value its proportion with respect to the quantity which has been really used.
To alleviate this drawback, it is known, particularly from patent application EP-0 626 259 to associate leakage-preventing stop devices with the lateral extremities of the scraping blade(s). However, the latter are oriented parallel to the displacement direction of the blades and the lateral overflow of the cream is not completely avoided, but only slowed down.